In order to make use of metals, they first must be extracted from raw materials, such as metal oxides, and thereafter separated from other materials either used for or generated by the extraction process. One particular problem is how to extract metals from materials while minimizing environmental contamination. For example, metals currently are liberated from metal oxides by first crushing the oxide and then treating the crushed material with an acid that dissolves the metals. Acid dissolution is followed by selective precipitation, electrowinning, or solvent extraction. Acid dissolution is unfortunately very nonspecific, and often produces many by-products, including aqueous and organic wastes, that create serious environmental problems in their own right and which must be separated from the metals.
The purification or separation of certain metals from other metals or impurities is critical in many instances. For example, for the commercial processing of uranium, iron is one common impurity that must be separated from the uranium. Further, uranium and plutonium are separated from fission products of spent nuclear fuel using a solvent extraction reagent. The uranium is subsequently separated from the plutonium.
Alternatively, metals may need to be separated from waste streams containing other metals, metalloids or impurities. For example, heavy metals may need to be separated from a waste stream for environmentally benign disposal of other waste in the stream.
It is known to extract metals from materials using supercritical fluid extraction. A supercritical fluid is typically one that is gaseous at ambient conditions, but which is maintained at a temperature and pressure above its critical temperature and pressure. Supercritical solvents can be used to extract organic materials, such as caffeine from coffee beans. U.S. Pat. No. 4,911,941 provides an example of supercritical carbon dioxide extraction of caffeine in which green coffee beans are moved periodically through an extraction vessel and contacted with continuously flowing supercritical carbon dioxide. U.S. Pat. No. 4,898,673 shows a similar system in which soluble materials are continuously extracted from solids using supercritical carbon dioxide. The soluble solids are circulated in a closed-loop pipeline with the supercritical fluid.
Various features of supercritical fluid extraction of metal and metalloids are disclosed in Wai et al.'s U.S. Pat., Nos. 5,356,538, 5,606,724, and 5,730,874; Wai et al.'s U.S. patent application, Ser. No. 08/686,422, entitled Fluid Extraction, filed Jul. 26, 1996; and United States Provisional Patent Application, Serial No. 60/056,749, entitled Method for Dissociating Metal-Ligand Complexes, filed Jul. Aug. 20, 1997. Wai's patents and applications, collectively referred to herein as Wai's patent documents, are incorporated herein by reference. Wai's patent documents disclose various features of extraction of metalloid and metal ions from materials by exposing the material to a fluid solvent, particularly supercritical carbon dioxide, containing a chelating agent.
Despite these prior known processes, there are still some disadvantages of traditional purification processes for metals, such as uranium. These disadvantages include: (a) low purified quantities of metal; (b) time consuming purification steps; and (c) the creation of undesirable waste streams.
A need therefore exists for an environmentally safe method for separating and/or purifying metals from other metals, metalloids and/or impurities. The need further exists for a method which is both efficient and which provides greater yields of the extracted and purified metals.